Do sticky traps work?

Q: Do sticky traps work?

Sticky traps work as a monitoring tool, not a cure. Yellow cards catch fungus gnats, whiteflies, winged aphids and leafminers; blue cards are somewhat more attractive to thrips. They catch only flying adults, so they never fix larvae in the medium or spider mites. Count catches weekly and replace the card: a rising count is your early warning, and a falling count shows the real treatment is working.

Short answer

Sticky traps work well for what they are: a counter of flying adult pests. Yellow cards catch fungus gnats, whiteflies, winged aphids and leafminers; blue cards pull somewhat more thrips. Counted weekly they give early warning, but they rarely control a pest by themselves. Established as a monitoring method.

Why

Insects fly to the colour. Many small flying pests are drawn to yellow, and thrips are drawn to blue and yellow ^[1]^[2].

Only adults are caught. Fungus gnat larvae live and feed in the top few centimetres of wet medium, so cards do nothing about the stage that chews roots ^[3]. Control comes from drier media plus Bti, nematodes or soil predatory mites ^[4].

Wingless pests are invisible to cards. Spider mites are found with a lens on leaf undersides, not on cards ^[5].

Trends beat totals. A count that climbs week on week is the signal to act; a count that falls over 2–3 weeks confirms a treatment is working ^[3].

The numbers

Pest	Card colour	Placement
Fungus gnats	Yellow	Flat on or just above the medium
Whiteflies, winged aphids, leafminers	Yellow	Just above the canopy, several per room
Thrips	Blue or yellow	Just above the canopy
Spider, broad and russet mites	Not caught	Use a 10–20x lens
Counting	Weekly	Count by type, record, replace

What to do

Hang one or two cards per small tent, more in rooms, from day one.
Put gnat cards low and foliar-pest cards just above the canopy.
Count weekly by pest type and write it down.
When counts rise, identify the pest and treat the cause: watering for gnats, predators for thrips.
Remove or reduce cards while releasing flying beneficials, which they also catch.

Common mistakes

Expecting cards to clear an infestation.
Leaving the same card up for months, so nobody knows what is new.
Looking for spider mites on cards.
Letting cards touch buds or hair.

Full article: Fungus gnats: the watering problem with wings
Answers: Preventing pests indoors · Small white bugs in soil
Articles: Thrips · Whiteflies
Terms: sticky trap · Bti

Follow-up questions

Will sticky traps get rid of fungus gnats?

Sticky traps will not get rid of fungus gnats on their own, because they catch only flying adults while the larvae keep feeding in the wet medium. Fix the watering first by letting the top 2–5 cm (1–2 in) of soil dry, then use Bti, Steinernema feltiae nematodes or soil predatory mites against larvae. Use yellow cards to count adults weekly and confirm numbers are falling.

Do sticky traps catch spider mites?

Sticky traps do not catch spider mites usefully, because spider mites are wingless and walk or drift between plants rather than fly to coloured cards. Find them with a 10–20x lens on leaf undersides, looking for oval mites about 0.5 mm long, round clear eggs and pale stippling on the upper leaf surface. A branch tapped over white paper also shows moving specks.

Where should I put sticky traps?

Place sticky traps for fungus gnats flat on or just above the surface of the medium, and traps for thrips, whiteflies and winged aphids just above the canopy, away from buds and hair. Use several in a room, because whiteflies do not fly far. Mark a counting area, count catches by type every week, write the number down and replace the card.